This invention relates to radio communications systems, methods of canceling interference in a radio communications system on a small platform, and specifically to an adaptive interference cancellation receiving system.
Self-generated interference imposes a major limit on radio communications system performance where many transmitters and receivers are crowded onto a relatively small platform such as a ship or aircraft. Traditional solutions have focused on RF bandpass filters, better equipment performance, and antenna isolation. No combination of these techniques today has sufficiently reduced cosite interference to acceptable levels.
Interference cancellation systems, such as that disclosed in U.S. Pat. No. 3,699,444, incorporated herein by reference, have been used, to minimize or eliminate interference in radio receivers. These adaptive interference cancellation (AIC) systems are very simple in theory, but very complex in practice. AIC systems sample the output of each collocated transmitter and shift phase and amplitude of a sampled transmitter signal so that when summed with a transmitter interference impinging signal on a victim receiver antenna, the two signals will exactly cancel, leaving the receiver (ideally) interference-free. While this technique is basic in principle and well-known in the radio art, it has been very difficult to implement in a complex multi-channel system due to the difficulty in achieving stable, fine-resolution variable phase shifting at a reasonable cost.
What is needed is a means and apparatus for implementation of an AIC system using a novel phase shift means.